Apparatus for preventing corrosion of the vessel of a storage tank

ABSTRACT

A method prevents corrosion of a vessel made of a corrodible material. The vessel has a bottom which has an outer surface. The method includes detecting the appearance of water at the outer surface of the bottom of the vessel.

This Patent Application is a division of co-pending patent applicationSer. No. 08/574,446, filed Dec. 15, 1995 entitled METHOD FOR PREVENTINGCORROSION OF THE VESSEL OF A STORAGE TANK, AND STORAGE TANK FORIMPLEMENTING IT.

BACKGROUND OF THE INVENTION

The present invention relates to methods for preventing corrosion of thevessel, made of a corrodible material, of a storage tank, in particularof buried storage tanks, as well as to storage tanks equipped withdevices for checking the vessel which use these methods.

Methods and devices are known for leak detection in a storage tank. Forexample the French Patent No. 1,551,051 which describes a vessel whichincludes an inner skin, a liquid-presence detector being arranged on theinner surface of the bottom of the vessel. This type of device exhibitsthe drawback, on the one hand, of not protecting the outer surface ofthe vessel from corrosion resulting from the moisture outside the vesseland, on the other hand, does not make it possible to detect theappearance of water as soon as it occurs in the intermediate space. Thedocument Patent Abstract of Japan, Vol. 8 Number 68 (page 264) of Mar.30, 1984, also describes a vessel surrounded by a concrete wall, arecess having been formed in the gap between the concrete wall and thevessel, in which a water detector is arranged. However, this deviceexhibits the drawback of not warning the user of the appearance ofmoisture as soon as it appears, but, as in the case of the devicedescribed in French Patent No. 1,551,051, only after a certain timeduring which the corrosion may already have progressed too far.

A method is known for checking the good condition of the vessel of aburied storage tank, which consists in observing the appearance ofcorrosion on the outer wall of the vessel by using a camera, the imagesfrom which are sent to a monitor which is not buried. This method of theprior art exhibits the drawbacks, on the one hand, of being veryexpensive and, on the other hand, of not giving a warning until aftercorrosion has occurred.

SUMMARY OF THE INVENTION

The invention alleviates the drawbacks of the prior art via a method forpreventing corrosion of the vessel of a storage tank which, while beingmuch less complicated to implement than the prior method, makes itpossible to warn the user even before corrosion of the storage tankappears, and thus makes it possible to preserve the storage tank for avirtually unlimited lifetime.

According to the invention, the method includes in detecting theappearance of water which condenses on a part of the outer surface ofthe bottom of the vessel. It has now been understood that, when thevessel contains a cold liquid, for example liquefied petroleum gas, theappearance of water which has condensed is a forewarning of corrosion.By attending to the vessel as soon as this warning appears, there is abetter chance of keeping the vessel in good condition than if attentionwere deferred until the corrosion had become a leak. However, it is onthe outer surface of the bottom of the vessel where moisture will firstappear, as it is there that the temperature is lowest.

An improvement to the method includes, on the one hand, in encasing theouter surface of the bottom of the vessel or a part of this surface witha leaktight casing which, with the outer surface of the bottom of thevessel, or a part of this surface, defines a leaktight enclosure whichis filled, via an inlet situated on the casing, with an anhydrous fluid(air, nitrogen or the like) and, on the other hand, in checking thepossible appearance of water on the part of the outer surface of thebottom of the vessel. The outer surface is thus exposed to a medium suchthat it should normally be impossible for moisture to appear there, thecause of the appearance of water, a forewarning of corrosion, and thereis an effective check that water is not appearing If, however, theleaktightness of the casing should be faulty, and moisture succeeds inentering the enclosure which is supposed to be leaktight, action can betaken before corrosion sets in. This method makes it possible totranslate the phenomenon of future corrosion into an appearance ofwater, which can be measured by a simple physical quantity such as anelectric current.

The invention also envisages a storage tank having a vessel made of acorrodible material, steel for example, and having a bottom and a casingmade of a non-corrodible material surrounding at least the bottom of thevessel, defining a leaktight enclosure between it and the bottom of thevessel, a detector of the presence of water being mounted on the outersurface of the bottom of the vessel.

The casing may be a non-corrodible material and may be less pressureresistant than the vessel. It is particularly made of plastic.

In order, in the atmosphere of the enclosure, to measure a moisturecontent which is extremely low, due to the very low temperatures whichthe storage tank reaches by reason of the evaporation of the liquefiedgas which it contains, an improvement to the invention consists infilling the leaktight enclosure with an anhydrous fluid (air, nitrogenfor example), thus making it possible to protect the outer surface ofthe vessel from the ambient moisture in the leaktight enclosure.Anhydrous fluid is understood to mean a fluid which contains no liquidwater, and the water vapour content of which is too low for condensationto occur.

In order to be able to measure traces of moisture, the detector of thestorage tank according to the invention may consist of a layer ofhygroscopic salt (chloride, nitrate, sulphate of alkaline metal or thelike), which is fixed onto the outer surface of the vessel by a knownfixing means (adhesive, crimping, etc) and with which two electrodes,made of a rustproof material (gold, platinum or the like) or plated witha material of this type, are placed in close contact at a distance fromone another.

The hygroscopic salt has very low conductivity in crystallized form,which becomes much greater when it is in solution. Electric currentcannot pass between the two electrodes as long as the hygroscopic saltis in crystallized form. When condensation water appears, the saltdissolves and current can pass between the two electrodes.

By linking the electrodes, by two conductors, to an information meansincluding an electric circuit comprising an electrical current sourceand a means of detecting the passage of the current, it is possible toknow that condensation water has appeared.

Generally, the vessel of a storage tank is made of a corrodible materialwhich conducts electricity. For current not to be able to pass betweenthe electrodes via the vessel of the storage tank, it is necessary toensure, when the electrodes are mounted in close contact with the layerof salt, that this layer of salt completely insulates the electrodesfrom the outer surface of the vessel.

An improvement to the detector of a storage tank according to theinvention consists in placing a heat-conducting but electricallyinsulating membrane between the outer surface of the vessel and thelayer of salt. This membrane makes it possible to provide for theelectrical insulation of the electrodes with respect to the outersurface of the vessel, while providing thermal conduction which ensuresthat the layer of salt is at a temperature equal to that of the outersurface of the vessel.

An improvement to the detector consists in placing a removable fixingpiece, particularly a magnet, between this membrane and the outersurface of the vessel.

By virtue of this magnet, the detector can easily be mounted under thevessel simply by sticking the magnet, which then supports the detector,against the vessel. Likewise, the detector can be withdrawn from thevessel simply by moving the magnet away from the storage tank.

In practice, it is convenient to have access easily to the data suppliedby the detector, despite the fact that detection is performed within theleaktight casing, preferably under the very bottom of the vessel, whichis itself buried.

An improvement to the invention makes it possible to resolve thisproblem by placing the information means outside the leaktightenclosure, in such a way that it is easily accessible by the user.

In order to enhance the resolution of the detector even further, that isto say to enhance the minimum quantity of water necessary for a currentto be visible on the means of detecting the passage of current,according to an improvement of the invention, a means intended toamplify the current which passes through the electrodes when the saltbecomes conducting is added to the electric circuit.

According to one embodiment of the invention, this amplification meansincludes a transistor (of 2 N 222 type, for example), the base of whichis linked to an electrode, while the branch consisting of the electriccurrent source and of the means of detecting the passage of the currentis mounted between the collector and the emitter, the collector alsobeing linked to the other electrode. Thus, when a current enters thebase of the transistor, it emerges therefrom amplified via the emitterbefore passing into the current passage detector.

According to an improvement of the invention, the detector is equippedwith a means of memory storage of the fact that current has passedbetween the electrodes and that, therefore, water has appeared on theouter surface of the vessel.

This memory storage allows the user of the storage tank to be informedthat water has appeared at a given moment without having to consult theinformation means at this precise moment. This memory-storage means isparticularly useful in the case in which the detector detects waterappearing although the user is not in the process of reading theinformation and although, before the user reads the information, thiswater disappears.

The electrical circuit of an embodiment of a detector equipped with sucha memory-storage means consists of a tripole, for example a transistor,having the property that, if current enters via a first pole, the dipoleconsisting of the second and third poles is conducting and that, ifcurrent does not enter via the first pole, the dipole is not conducting,the first pole being linked to an electrode, one pole of the second andthird poles being linked to the other electrode, of a memory-storageswitch controlled by the current, initially in open position and whichpasses into closed position when current leaves the pole of the tripolewhich is not linked to an electrode, of an electric current source andof a current passage detection means, the pole of the tripole not linkedto an electrode being linked to one of the electric current sourceterminals and to one of the terminals of the switch, the other terminalof the switch being linked to the means of detecting the passage ofcurrent, which is itself linked to the pole of the dipole which islinked to an electrode and to the other electric current sourceterminal.

In this layout, the memory storage means consists of the memory-storageswitch controlled by the current. The lamp serving as a means ofdetecting the passage of the current then remains continuouslyilluminated, being supplied by the electric current source.

In practice, the electric current source will rapidly discharge, if itis left thus continuously on load. If this discharge takes place beforethe user reads the information, the lamp will be extinguished due tothis discharge, and he will wrongly conclude that the storage tank isoperating appropriately.

In order to protect the electric current source from such a discharge,an improvement to the invention consists in placing, in the electricalcircuit, a means intended to prevent the electric current sourcesupplying current immediately after a current has been detected.

An embodiment of the electrical circuit equipped with such a means is asfollows: it consists of a tripole, for example a transistor, having theproperty that, if current enters via a first pole, the dipole consistingof the second and third poles is conducting and that, if current doesnot enter via the first pole, the dipole is not conducting, the firstpole being linked to an electrode, one pole of the second and thirdpoles being linked to the other electrode, of a memory-storage switchexcited by the current, initially in open position and which passes toclosed position when current leaves the pole of the tripole which is notlinked to an electrode, of an electric current source and of a currentpassage detection means, the pole of the tripole not linked to anelectrode being linked to one of the electric current source terminalsand to one of the terminals of the switch, the other terminal of theswitch being linked to the means of detecting the passage of current,itself linked to the pole of the dipole which is linked to an electrodeand to the other electric current source terminal, a non-supply switchwith two positions, open and closed, initially in closed position andwhich passes to open position when it is excited by current beingmounted between the pole of the tripole which is not linked to anelectrode and the input of the first two-position switch controlled bythe current and a manual switch controlling the passage of the currentin the means of detecting the passage of current.

By virtue of this layout, the electric current source supplies currentonly during the very short period of time between the moment when thesalt becomes conducting and the moment when the memory-storage switchopens.

The advantage of this layout is that, after water condensation has beendetected, the current source no longer supplies current but the user isnevertheless informed that water has appeared during his absence. Thisis because the memory-storage switch controlled by the current hasactually closed and, when the user closes the manual switch, theelectric current source again feeds into the current passage detectionmeans and warns the user that current has passed in the circuit duringhis absence.

Another embodiment includes replacing the two switches excited by thecurrent by a bistable relay with two branches, each branch having twoopen and closed positions, the first being initially in closed positionand the second initially in open position, and the first opening and thesecond closing when the current enters the bistable relay.

Thus, when the salt becomes conducting, current enters thememory-storage switch excited by the current (the branch of the relay),which is initially closed, which has the effect of opening it, whereasthe second switch (the second branch) closes.

It can also happen that the detector becomes detached from the outersurface of the vessel.

An improvement to the detector of the storage tank according to theinvention consists in equipping this detector with a means indicatingthe fact that it has become detached from the vessel.

An embodiment of a detector equipped with such an indicator meanscomprises, as indicator means, a switch with a spring tensioned betweenthe outer surface of the vessel and an end stop integral with thedetector, and conductors linking it to each electrode.

Hence when the detector and its integral end stop move away from thesurface of the vessel, the spring-type switch relaxes and closes so asto allow current to pass. Everything then happens as if the detector haddetected an appearance of water, and the user is warned by the means ofdetecting the passage of the current that an anomaly has appeared in thedetector.

When the detector does not detect anything, this may be because no watercondensation appears, but also because the information means isdefective.

In order to supply the user with a signal warning him of an anomaly inthe information means, an improvement to the storage tank according tothe invention consists in two conductors linked to the outside of theleaktight casing by a verification switch, and each linked, within thecasing, to one of the electrodes. This makes it possible, when theverification switch is closed, to make it seem that water is appearingon the outer surface of the vessel and thus to verify that theinformation means is operating normally. If such is not the case, theuser knows that an anomaly exists and that attention is necessary.

A variant of a storage tank comprising a device for warning the userthat an anomaly is impeding its correct operation consists of a detectorwhich comprises a hygroscopic salt, two electrodes linked together by aresistor, of value less than that constituted by the undissolved saltand greater than that constituted by the dissolved salt, and preferablya sheath made of synthetic material chosen for its properties of highwater absorption and low water desorption. A non-limiting example of anappropriate material is a polyamide (nylon type).

When condensation occurs on the outer surface of the vessel of thestorage tank, the condensed water easily penetrates the material of thesheath and alters the resistance of the hygroscopic salt, bringing it toa value less than that of the resistor placed between the twoelectrodes. In this way, observation of the presence of condensation canbe verified by a simple resistance measurement.

Three cases can arise:

1) the value of the overall resistance is greater than that of theresistor placed in series with the electrodes: the apparatus is faulty,as a consequence of a break in circuit,

2) the value of the overall resistance corresponds to that of theresistor placed in series with the electrodes: the sensor is operatingand there has been no condensation;

3) the value of the overall resistance is less than that of the resistorplaced in series: there has been condensation.

A simple ohmmeter allows checking both of the circuit and of thepresence of condensation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawing given solely by way of example,

FIG. 1 is a diagrammatic view in section of a storage tank according tothe invention,

FIG. 2 is a diagrammatic view in section of a preferred embodiment ofthe detector according to the invention,

FIG. 3 is a diagram of a preferred embodiment of the electrical circuitof the storage tank according to the invention, corresponding to thepart which is outside the leaktight casing,

FIG. 4 is a diagram of another possible embodiment of the detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The storage tank represented in FIG. 1 is designated overall by thereference 1. It includes a vessel 2 made of steel surrounded by a casing3 made of a non-corrodible material, for example a plastic, such aspolyethylene, which, with the outer surface 4 of the vessel 2, defines aleaktight enclosure 5 the leaktightness of which is ensured by sealingjoints 6. An inlet (not represented), equipped with a stopper, makes itpossible to introduce anhydrous fluid or gas into the enclosure 5.

Under the bottom of the vessel 2, is mounted a detector 7 of theappearance of water linked by conductors 8, 9 to an information means 10making it possible to know whether the detector 7 has detectedcondensation This information means 10 is easily accessible to the userof the storage tank, being close to the surface of the ground S.

The detector (FIG. 2) consists of a membrane 11, electrically insulatingand thermally conducting, for example SILPAD 400 manufactured by theBERGQUIST company, one of the faces of which is in permanent contactwith the outer surface 4 of the vessel by means of a thermallyconducting magnet 12, while the other face 13 is covered by a layer 14of hygroscopic salt (LiCl, NaCl or the like), the conductivity of whichin solution is much greater than in crystallized form. In this layer aremounted two electrodes 15, 16 made of a rustproof material (gold,platinum or the like), arranged in close contact with the face 13 of themembrane 11. A plastic box 17 covers over the inner part of theenclosure of the detector in order to protect the layer of hygroscopicsalt, and can serve as a reserve of hygroscopic salt.

A spring-type switch 18 has its spring 38 mounted compressed between thebottom of the box 17 forming an end stop and the outer surface 4 of thevessel, with the interposition of a pusher 39 equipped with a blade 40.When the spring 38 is relaxed because the bottom 17 moves away from thesurface 4, the conducting blade 40 comes into contact with two terminals41, and is linked by two conductors 19, 20 to each of the electrodes.Two conductors 8, 9 link the electrodes to the information means 10after having passed through the leaktight casing 3 via sealed orifices.

This information means 10 comprises an electrical circuit 21 connectedto the terminals of the conductors 8, 9 coming from the electrodes.

The conductor 8 is linked to the base of a transistor 22, the emitter ofwhich is linked to a first branch 23a of a bistable relay 23 with twopositions NC and NO. This bistable relay branch 23a is linked by a wire,on the one hand, to a terminal of an electric current source 24 and, onthe other hand, to the second branch of the bistable relay 23b with twopositions NC and NO. The latter is linked by a wire to a lamp 25,serving as a means for detecting the passage of a current, the lampitself being linked to a manual switch 26 itself linked to the collectorof the transistor 22. The second electric current source terminal ofsource 24 is also linked to the collector of the transistor 22. A branchcomprising a zero-reset switch 27 and an auxiliary coil 28 is mountedbetween the electric current source terminal which is also linked to thebistable relay 23. The coil 29 forms part of the bistable relay 23.

Two other conductors 30, 31 are mounted at the terminals of theelectrodes 15, 16 and linked, after having passed through the casing 3in a leaktight way, by a verification switch 32.

The liquid or gas to be stored in the vessel of the storage tank, forexample a liquefied petroleum gas, is introduced via an inlet 33, and agas or an anhydrous fluid is introduced into the leaktight enclosure 5.The leaktightness of the enclosure 5 makes it possible to ensure thatthe outer surface 4 of the vessel 2 is never exposed to a moistatmosphere, which protects it against corrosion. If, nonetheless,moisture succeeds in getting into the enclosure 5, then, by reason ofthe low temperature of the liquefied petroleum gas which is in the lowerpart of the inside of the vessel, it will happen that condensation waterwill appear on the outer surface of the vessel, and in particular in thebottom of the vessel where the temperature is lowest. This appearance ofwater is detected by the water appearance detector 7. Via the conductors8, 9, the information that water has been detected is sent to the lamp25 which allows the user to know that attention is necessary in order toavoid future corrosion of the vessel 2.

Under normal conditions (no appearance of water), the conductivity ofthe hygroscopic salt between the two electrodes 15, 16 is very low andno current passes in the conductors 8, 9, all the more so as twohigh-value resistors 34, 35 are mounted in series respectively with eachelectrode.

If water appears on the layer 14 of salt of the detector 7, itsconductivity increases greatly and a flow of current ensues, whichenters the base of the transistor 22. The current emerges amplified fromthe emitter of the transistor and enters the first branch 23A, with twobistable states, of the bistable relay which then passes from the NC(closed) state, which allows the current to pass, to the NO (open) statewhich prevents the current passing. The current which has just passedinto the first branch of the bistable relay also enters the secondbranch 23B of the bistable relay which itself passes from the NO stateto the NC state. The circuit consisting of the electric current source24, of the lamp 25 and of the manual switch 26 is then closed, andcurrent passes through the lamp 25 if the switch 26 is closed. The user,by closing this switch 26, is then informed:

if the lamp lights, then current has passed between the electrodes sincethe last time the bistable relay 23 was in the normal position (branch Aclosed, branch B open), (generally the last inspection);

if the lamp does not light and lights when he closes the verificationswitch 32, then no current has flowed since the last time the bistablerelay was set to normal position.

The switch 32 short-circuits the electrodes and allows current to passbetween them. By closing it, the user can then verify that theinformation means 10 is operating normally, if the lamp lights. If itdoes not light, it is because a fault exists in the information means 10(defective cable, defective components, etc) and attention is requiredthereto.

Once these checks have been carried out, the user reopens the manualswitch 26 and closes the zero-reset switch 27 for a brief moment, whichresets the bistable relay into its normal position, by supplying powerto the auxiliary coil 29 of the relay.

By virtue of the switch 26 and of the branch 23a, the electric currentsource is not loaded throughout the whole time between the instant whenwater appeared and the instant when the user carries out his operations.This makes it possible, from then on, to ensure a very long lifetime forthe battery.

When the user closes the switch 26 and when the lamp lights, thissignals an anomaly. Condensation has been able to occur, as wasdescribed above. Another cause is that the detector 7 has becomedetached from the surface of the vessel 2. This is because, if thisshould occur, the spring-type switch 18 has its spring decompressed,thus bringing the switch to the closed position. Current then passesbetween the two electrodes and it applies voltage to the lamp 25 in thesame way as if condensation had appeared.

Hence, when upon pressing on the switch 26, the user sees the lamp 25light, he knows that attention is necessary in the leaktight enclosurein order either to dry out the enclosure, or to refix the detector tothe vessel.

In FIG. 4, a resistor 36 is mounted between the electrodes (15, 16) inthe hygroscopic salt 14 within a sheath 37 made of a material whichstrongly absorbs water and has low desorption.

When condensation occurs on the outer surface of the vessel of thestorage tank, the condensed water easily penetrates the material of thesheath and alters the resistance of the hygroscopic salt, bringing it toa value less than that of the resistor 36. In this way, observation ofthe presence of condensation can be verified by a simple resistancemeasurement.

Three cases can occur:

1) the value of the overall resistance is greater than that of theresistor 36: the detector 7 is defective following a break in circuit,etc.

2) the value of the overall resistance corresponds to that of theresistor 36: the detector 7 is operating and there has been nocondensation.

3) the value of the overall resistance is less than that of the resistor36: there has been condensation.

I claim:
 1. A storage tank comprising a vessel, a casing surrounding atleast a part of the vessel and defining with this said part anenclosure, and a detector being disposed in the enclosure, whereininformation means for providing information are disposed outside of thecasing, are linked to the detector, and are provided with means formemory-storage of the apparition of water detected by the detector,wherein the memory-storage means stores the detection of the apparitionof water after the water disappears.
 2. The storage tank as claimed inclaim 1, wherein the information means comprise an electrical circuitsupplied by an electric current source and means for preventing theelectric current source from supplying current after the detector hasdetected said apparition of water.
 3. The storage tank as claimed inclaim 2, wherein the electrical circuit of the information meanscomprises, as memory-storage means, a means for remembering that currenthas passed therein, the electrical circuit comprising a tripole with afirst pole, a second pole and a third pole, the third pole and thesecond pole constituting a dipole, such that if current enters via thefirst pole, the dipole is conducting and that, if no current enters viathe first pole, the dipole is not conducting, the first pole of whichbeing linked to an electrode, one pole of the second and third poles islinked to another electrode, a memory-storage two positions switch withtwo terminals and an input, the positions being a closed position and anopen position, which allows and does not allow current to passrespectively, initially in open position and which passes to closedposition when a current leaves the pole of the tripole which is notlinked to an electrode, an electric current source with two terminalsand a means for detecting the passage of the current, the pole of thetripole which is not linked to an electrode being linked to one of theterminals of the electric current source and to one of the terminals ofthe switch, the other terminal of the switch being linked to the meansof detecting the passage of current, itself linked to the pole of thedipole which is linked to an electrode and to the other terminal of theelectric current source.
 4. The storage tank as claimed in claim 3,wherein the electrical circuit also comprises a second non-supply,two-position switch, controllable by the current, initially in closedposition and passing to open position when a current leaves the pole ofthe tripole which is not linked to an electrode, mounted between thepole of the tripole which is not linked to an electrode and the input ofthe first two-position switch, and a third switch controlling thepassage of the current in the means for detecting the passage ofcurrent.
 5. The storage tank as claimed in claim 2, in which a means isprovided for verifying the correct operation of the information means.6. The storage tank as claimed in claim 5, in which the verificationmeans comprise two electrodes, two conductors, each conductor startingfrom the conductor's own of the two electrodes and a verification switchmounted between the two conductors.
 7. The storage tank as claimed inclaim 5, in which the verification means comprise two electrodes, twoconductors, each conductor starting from the conductor's own of the twoelectrodes and a resistor mounted in series between the two electrodes.8. The storage tank as claimed in claim 3, wherein the electricalcircuit also comprises a bistable relay with a first branch and a secondbranch, each said branch having two stable states, open and closed, thefirst branch being initially in closed position and the second branch inopen position, the first branch passing to open position when a currentleaves the pole of the tripole which is not linked to an electrode andthe input of the first two position switch and the second branchcontrolling the passage of the current in the means for detecting thepassage of current.
 9. The storage tank as claimed in claim 5, in whicha means is provided for verifying the correct operation of theinformation means.
 10. The storage tank as claimed in claim 3, in whicha means is provided for verifying the correct operation of theinformation means.
 11. The storage tank as claimed in claim 10, in whichthe verification means comprise two electrodes, two conductors, eachconductor starting from the conductor's own of the two electrodes and averification switch mounted between the two conductors.
 12. The storagetank as claimed in claim 10, in which the verification means comprisetwo electrodes, two conductors, each conductor starting from theconductor's own of the two electrodes and a resistor mounted in seriesbetween the two electrodes.
 13. The storage tank as claimed in claim 4,in which means is provided for verifying the correct operation of theinformation means.
 14. The storage tank as claimed in claim 13, in whichthe verification means comprise two electrodes, two conductors, eachconductor starting from the conductors own of the two electrodes and averification switch mounted between the two conductors.
 15. The storagetank as claimed in claim 13, in which the verification means comprisetwo electrodes, two conductors, each conductor starting from theconductors own of the two electrodes and a resistor mounted in seriesbetween the two electrodes.
 16. The storage tank as claimed in claim 4wherein there is provided a means for indicating that the detector is nolonger in a thermal conduction relation with the said part of thevessel, said indicating means comprising, an indicator switch, linked tothe electrodes via two auxiliary conductors, said indicator switchclosing itself when said detector moves away from the part of thevessel.
 17. The storage tank as claimed in claim 1, in which means isprovided for verifying the correct operation of the information means.18. The storage tank as claimed in claim 17, in which the verificationmeans comprise two electrodes, two conductors, each conductor startingfrom the conductor's own of the two electrodes and a verification switchmounted between the two conductors.
 19. The storage tank as claimed inclaim 17, wherein the information means are not buried.
 20. The storagetank as claimed in claim 17 wherein the memory-storage means are notburied.
 21. The storage tank as claimed in claim 17, wherein the meansfor verifying the correct operation of the information means are notburied.
 22. The storage tank as claimed in claim 17, wherein theinformation means and the memory-storage means and the means forverifying the correct operation of the information means are not buried.23. The storage tank as claimed in claim 17, in which the verificationmeans comprise two electrodes, two conductors, each conductor startingfrom the conductor's own of the two electrodes and a resistor mounted inseries between the two electrodes.